Spraying technology (bore spraying) is known that sprays metal or ceramics to deposit coatings (sprayed coatings) on internal surfaces of cylinder bores of cylinder blocks installed in vehicles, and then implements honing processing thereon to form smooth sliding surfaces.
Since a sprayed coating is provided such that liquid droplets are applied to a surface in layers, the sprayed coating typically creates a porous structure including a plurality of pores and minute defects derived from iron oxide, for example. The porous structure may lead to minute surface defects because part of a surface layer of the sprayed coating comes off when after treatment such as honing is performed after spraying. In addition, the presence of casting cavities on the internal surface of the cylinder bore also leads to surface defects because a sprayed coating is not appropriately applied to the surface. While such surface defects serve as an oil collector and contribute to an improvement of lubrication of a piston linkage, the consumption of oil increases when the volume of defects is excessively large. It is therefore preferable to detect the surface defects on the internal surface of the cylinder bore and regulate the upper limit of the volume of the surface defects.
A typical method of detecting surface detects on an internal surface of a cylinder bore is to image the internal surface of the cylinder bore with, for example, a CCD camera to obtain a two-dimensional image, and detect the number and size of the surface defects from the two-dimensional image (for example, see Japanese Patent Application Publication No. H11-023477).
Japanese Patent Application Publication No. H11-023477 detects the number and size of the surface defects from the two-dimensional image, while the information about the depth of the defects is not obtained. Thus, a difficulty lies in measuring the volume of the surface defects on the internal surface of the cylinder bore with high accuracy.